The long-term aim of the studies proposed in this application is to understand how the four different members of the epidermal growth factor (EGF) receptor, or erbB, family are activated. X-ray crystallography will be used to determine how EGF induces dimerization of its receptor, and this will provide a framework to investigate mechanistic differences between ligand-induced erbB receptor homo- and hetero- dimerization. A detailed structural understanding of receptor activation will provide the basis for designing novel approaches for reversing inappropriate activation in human cancers. We hope, for example, that this will lead to the development of the next generation of erbB2-directed breast cancer treatments. Receptor tyrosine kinases of the EGF receptor family have been implicated in several human cancers. In particular, the second member that is known as erbB2, HER-2, or the Neu oncogene product, is found overexpressed in 30 percent of human breast cancer cases. ErbB2 is the target of several therapeutic strategies, and humanized monoclonal antibodies raised against erbB2 (Herceptin) are used clinically to treat breast cancer. ErbB2 and its related receptors are thought to contribute to development of breast and other cancers by being activated inappropriately, usually through their over-expression, in turn promoting uncontrolled cell growth. Like other cytokine and growth factor receptors, the erbB receptors are normally activated by homo- and/or hetero-dimerization induced upon ligand binding. A detailed understanding of how growth factor binding controls erbB receptor homo- and hetero-dimerization will be very important for developing new approaches to control receptor activation when it goes awry. While many growth factors are dimers, and can bind simultaneously to two receptor molecules, the erbB ligands are monomeric and have not been shown to be bivalent. Our previous biophysical studies have shown that the extracellular domain (ECD) of the EGF receptor dimerizes completely upon EGF binding, and forms a 2:2 EGF:receptor complex. We have crystallized this complex, and propose to determine its structure by X-ray crystallography. We have also examined the homo- and hetero-oligomerization properties of the other erbB family members. While neuregulin (NRG1-beta1) binds to both the ECD of erbB3 and erbB4, it induces dimerization only of the erbB4 ECD. We will extend our crystallographic analysis to these other complexes.